The human brain is an exceedingly complex processing system, which integrates continual streams of incoming sensory input data with stored memories, uses the input data and memories in complex decision processes at both conscious and unconscious levels and, on the basis of these processes, generates observable behaviors by activation of its motor or movement control pathways and the muscles which these innervate. The neurons of the nervous system propagate input data by generating characteristic electrical pulses called action potentials, or neural spikes, that can travel along nerve fibers. A single neuron or a group of neurons represent and transmit information by firing sequences of neural spikes in various temporal patterns. Information is carried in the neural spike arrival times.
In certain cases of traumatic injury or neurological disease, the brain can be partially isolated from the periphery. Input data from certain senses are thus lost, at least for a portion of the body, as are many voluntary movements. Spinal cord injury is a well-known example of traumatic injury. With spinal cord injury, the pathways that link higher motor centers in the brain with the spinal cord and that are used for control of voluntary movements can be functionally transected at the site of injury. As a result, the patient is paralyzed, and can no longer voluntarily activate muscles that are innervated by regions of the spinal cord below the level of the injury. Despite the injury to their long fibers, however, many of the cells in these higher brain regions that control voluntary movement will survive and can still be activated voluntarily to generate electric signals for controlling voluntary movement. By recording the electrical activities produced from these cells with implantable neural sensors (e.g., a microwire electrode array, a microwire, a magnetic field detector, chemical sensor, or other neural sensor), signals generated by the cells can be “exteriorized” and used for the control of external prostheses, such as an assist robot or an artificial limb, or functional electrical stimulation paralyzed muscles. Additionally, these generated signals can be used for control of computer operations such as the movement of a cursor on a computer display.
Another example of such loss occurs in cases of amyotrophic lateral sclerosis (Lou Gehrig's Disease), in which the motor neurons that control muscles, as well as some of the brain cells that control these motor neurons, degenerate. In advanced stages of this disease, the patient might have completely intact senses and thought processes, but is “locked in,” so that neither movements nor behavioral expressions of any kind can be made. Providing these patients with some way of communicating with the external world would greatly enhance their quality of life.
In sum, there is a need to develop a system for monitoring and processing the electrical signals from neurons within the central nervous system, so that the brain's electrical activity can be “exteriorized” and used for the voluntary control of external prostheses or assist devices which are adapted to provide sensory feedback. In this way, damaged pathways can be circumvented and some control of the environment can be restored; additionally, a patient can be provided the ability to interact with his or her environment. Because the electrical fields of small groups of neurons drop off rapidly with distance from the cells, a representative system can include surgically implanted electrodes or other neural sensors, which can be placed in close proximity to the individual or large numbers of brain cells that generate command signals for voluntary movement.
Neural signals can be detected by measuring the electric field potential of an area or region of the brain or other organ. The field potential detected at any one point represents the sum of the potential created by a number of electric potential generators in the area surrounding the field potential measuring device. By way of example, when an individual monitors a field potential (e.g., the amplitude of a field potential) at a point on the surface of the cerebral cortex, for example, what is detected is the overlapping summation of electric fields generated by active neurons in the depths of the cerebral cortex, which have spread through the tissues and up to the surface. These nerve cells can be characterized as point dipoles that are oriented perpendicular to the surface of the cerebral cortex. In other words, each cell or group of cells has a current source where positive charge moves outwardly across its membrane and a current sink where the same amount of positive charge moves inwardly at each instant. Thus, the flow of current across each cell or group of cells establishes an electric field potential that is equivalent to the electrostatic field potential of a pair of point charges, one positive at the location of the current source and one negative at the current sink. The amplitude of this field potential, i.e., the electric field strength, decreases inversely with distance in all directions from each point charge, and is relatively low at the surface of the cerebral cortex.
When many nerve cells are generating field potentials in a given region, these field potentials sum and overlap in the neural tissue, in the extracellular fluid, and at the brain surface. This summation is a linear function in this volume conductor, since the field strength of a given cell or group of cells varies inversely as a function of the distance from each current source or sink. Thus, if the electric potential of a given region of neurons is measured at a sufficient number of points and depths, it is possible to deduce the locations and amplitude of each dipole generator at any instant of time.
Integrated circuits, called neurochips, have been developed to acquire neural signals from a subject and condition the signals for processing. Some current neurochips include multi-channel sieve electrodes for detecting neural signals from regenerated axons. A sieve electrode is a planar structure with small throughbores extending therethrough. In order to implant a sieve electrode, an axon is severed, the ends placed through adjacent throughbores, and the nerve is allowed to heal. Signals in the regenerated axon are detected by the sieve electrode. Detected signals are then processed and transmitted by the neurochip for further processing.
Many current neural signal systems utilize radio frequency telemetry for transmitting information signals. A significant amount of the total power required for operating a neurochip is used to implement telemetry. High power consumption is undesirable for neurochips in order to achieve reduced neurochip and system size. Thus, neurochip telemetry and transmission methods are desired having lower power requirements for transmitting information signals. In general, double the power is required to transmit twice the amount of data. Thus, neurochips are desired that require as little data transmission as possible, thus using a lower amount of power to transmit. Further, neurochips are desired having a smaller size and improved circuitry for receiving, conditioning, and processing neural signals. Such improvements will reduce the amount of information that must be passed to through the telemetry links to other parts of the device thus conserving power and will distribute the processing burden to multiple devices operating serially and synchronously.